Erectile dysfunction (ED) is defined as the inability to achieve or maintain an erection sufficient for satisfactory sexual performance. The prevalence in United States is 10 to 20 million men. The male erectile response is a neurovascular event reliant on the complex interaction between neurological and vascular responses. Erectile dysfunction is multifactoral and has been typically classified by the primary presumed cause: vasculogenic, psychogenic, neurogenic, and endocrinologic disease. Any condition or injury that impairs the transmission of impulses along the psychogenic or reflexogenic neurological pathway, may be associated with neurogenic erectile dysfunction.
The penis is innervated by the dorsal penile and perineal nerves. These nerves are a continuation of sympathetic and parasympathetic autonomic nerves as well as sensory and motor somatic nerves. The somatic sensory system is responsible for the specialized structures that transmit information about the external environment. There are four major classes of somatic sensation: pain, temperature, position sense, and touch-pressure sensation. These stimuli are transmitted in the autonomic nervous system through both large ( and ) and small ( and C) caliber nerves.
Currently, as many tests are available to evaluate the sensory afferent nerves from the penile skin, as well as the motor efferent nerves to the perineum. These tests include the bulbocavernosus reflex, penile thermal sensory threshold measurement, corpus cavernosum electromyogram (CC-EMG) signal assessment, somatosensory evoked potentials, anal or urethral sphincter EMG, and vibration perception sensitivity. The problems with many of these tests is that they tend to be complex, time consuming, and do not directly measure autonomic function or correlate with the degree of ED.
The study leading to the present invention was designed to test the hypothesis that autonomic neuropathy is a significant component of ED, and further, that this deficit could be evaluated by measuring specific aspects of sensation in the nerves innervating the penis. This study was also designed to evaluate the impact of age and concomitant medical conditions, such as diabetes on the loss of cutaneous sensation of the penis.
In recent years, quantitative sensory testing (QST) has emerged as an important adjunct of the neurologic examination and its use has been recommended by several consensus panels including: American Neurologic Association, American Diabetes Association and the Peripheral Nerve Society. A variety of QST instruments and testing algorithms have been developed and utilized to provide standardized, non-invasive and semi-objective measures of neural function. These procedures have proven valuable in tracing the onset and progression of peripheral neuropathy associated with aging, disease, exposure to exogenous neurotoxins and in documenting iatrogenic neuropathies associated with the treatment of cancer and HIV infection. Validated equipment and procedures exist for the testing of vibration, pressure, spatial perception, warm, cold and painful stimuli. The approach is to provide well controlled, standardized sensory stimuli and to evaluate detection threshold using established psychophysical procedures, such as ascending method of limits, and two alternative forced choice. For instance, a simple hand-held device, the Semmes-Weinstein monofilament has been found a valuable method of screening for deficits in pressure sensation, while the Tactile Circumferential Discriminator has been used to screen for neuropathy and foot ulcer risk.
While most tests of QST have focused on sensation in the hands and/or feet, a few studies have used this approach to evaluate sensation in the genital region. Romanzi et al. found that Semmes-Weinstein monofilaments could be used to evaluate pressure/touch sensitivity of the female external genitalia and various devices have been used to measure vibration thresholds at the penis. A recent review of 13 studies on vibrotactile penile thresholds of men with ED were significantly higher (diminished sensitivity) than age-controlled functional males. Yarnitsky et al. found that penile thermal thresholds could be used as a repeatable, valid diagnostic tool to evaluate somatic small fiber function and reported normative values. Lefaucheur expanded the use of penile thermal thresholds, and demonstrated higher thresholds in impotent diabetic males.